Study On The Optimized Selection Of Deep Coalbed Methane Targets For Exploration And Development In Mid-eastern Qinshui Basin

China has abundant deep coalbed methane(CBM)resources that have not achieved commercial exploitation due to the geological nature of high ground stress,high reservoir pressure and temperature,low reservoir permeability,etc.Determining the development order of target areas is an important issue in the decision-making process of coalbed methane exploration and exploitation.This process is influenced by a series of factors,including the resource abundance,geological settings,development risk,economic efficiency etc.These factors contain conflicting requirements and have different dimensional parameters,which make traditional decision-making methods of single objective useless in dealing with this kind of synthetic-valuation problem.Previous research into the CBM target-area ranking mainly focus on the geological settings rather than the economics,with favorable areas selected on the basis of resource abundance and geological conditions.There is no comprehensive model of CBM-target-area ranking on the basis of geological evaluation,techno-economic analysis,and risk assessment.In light of this,a multi-attribute decision making model was developed to deal with the ranking and optimization problem of target areas with different resource types,geological conditions,development risks and overall benefits.The Yushe-Wuxiang block in mideastern Qinshui Basin was selected as the study subject,in which several geological units are ranked on the basis of geological-feasibility investigation,development-unit division,development-technology selection and the development-program optimization.Main findings of this study are concluded as follows:(1)The geological feasibility and potential for developing deep coalbed methane resources were identified.In addition,the block was divided into several units based on geological conditions and technologies for CBM development were then selected respectively.The study area was investigated to have large potential for CBM accumulations.The target No.3 and No.15 coals are deeply buried with depth greater than 1000 m,and these reservoirs are characterized by under pressure,low permeability,high gas content and moderate development potential.Six geological-unit types of CBM development are divided,including type ?(middle-shallow layer with moderate gas content),type ?(middle-shallow layer with medium-high gas content),type ?(middle-deep layer with medium-high gas content),type ?(middle-deep layer with high gas content),type V(deep layer with high gas content)and type VI(deep layer with extra-high gas content),which show variations in the burial depth,gas-bearing condition,physical properties of the reservoir,structural complexity and resource abundance etc.The No.3 coal is only adaptable to be exploited by fractured vertical wells.The type ?,?,?,and ? units of the No.15 coal are suitable for both fractured vertical wells and single horizontal wells,whereas the type V and VI units are only adaptable to fractured vertical wells.(2)Production of different development schemes in varying geological units were predicted,and the descending sequence of geological parameters in affecting the well performance in deep CBM reservoirs was identified.Due to the advantage in resource reserves,the production of the No.15 coal in the vertical well scheme are much higher than that of the No.3 coal.Type ? and type ? units have the best production performance,followed by type ? and type VI units,whereas type ? and type V units,have the lowest gas production.Gas recovery of the Type I–? units are higher than type V and VI units.Low permeability is the key factor constraining the well performance in deep CBM reservoirs,in which the high gas content,however,plays an active role in improving the gas production.The sequence from the highest to the lowest in cumulative gas recovery of the mixed-well scheme and the horizontal-well scheme are type ?,type ?,type ? and type ? units.Better single-well performance was found in fractured horizontal wells than nonfractured horizontal wells and fractured vertical wells.Cumulative gas production of the fractured horizontal well was highest at the type ? unit,followed by type ?,type ?,and type ? units,respectively.Permeability,Langmuir volume,gas content,Langmuir pressure,fracture porosity and thickness are key factors identified in affecting the production of deep coalbed methane wells with a descending order.(3)The CBM-development schemes in different geological units were optimized and the development order of six geological units was established on the basis of economic efficiencies.In addition,advices for improving the economic performance of developing deep CBM resources were proposed.The results of economic evaluation show that exploiting the No.3 coal will not be economically viable at the present economic and technical conditions.The descending sequence of economic performance for exploiting the No.15 coal is type ?-type ?-type ?-type VI-type ?-type V.The vertical-well scheme has better economic performance than mix-well scheme and horizontal-well scheme.The mix-well scheme in mediumshallow geological units(type ? and type ?)achieves better economic benefits than the horizontal-well scheme,but has poor economic performance in middle-deep units(type ? and type ?).Under the current technical and economic situations,it would be realistic and useful to improve the economics of deep-CBM development by properly enhancing the subsidy and granting more preferential tax policies.(4)An MADM model for CBM target-area ranking was established and was then applied to adjust the development order of geological units in the study area.The attribute set of the MADM model contains a total of ten factors,such as resource abundance,cumulative gas recovery,development-risk index and NPV etc.,which reflect the conditions of resource basis,development risks,economic performance and social benefits.The CBM-development order of “type ?-type ?-type VI-type ?-type ?-type V”was obtained on the basis of combination weighting and TOPSIS model.Compared with the development order obtained by economic evaluation,the optimized order derived from MADM model inherits the aim of economic-performance maximization,and also reflect compromises of the resource conditions,development risks and social benefits etc.,the result of MADM is more in accordance with the fact of CBM development and the sustainable development of resources.